There are many known types of welding-type power supplies that provide welding type power. Welding type power, as used herein, refers to power suitable for electric arc welding, plasma arc cutting or induction heating. Welding-type power supply, as used herein, refers to a power supply that can provide welding type power. Welding type systems are used to perform a variety of processes and used in a variety of settings. Welding-type system, as used herein, is a system that can provide welding type power, and can include control and power circuitry, wire feeders, and ancillary equipment.
Some welding type systems include a preregulator which provides a dc bus, followed by a converter based output circuit. The preregulator conditions the input power, and provides a known dc bus. The inverter based output circuit receives the bus and provides the welding type power as an output. One successful design includes a boost circuit as part of the preregulator, and the output circuit includes an inverter, transformer, rectifier and output inductor. This type of welding type power supply is described in U.S. Pat. No. 6,987,242 (Geissler) and U.S. Pat. No. 6,115,273 (Geissler). Another welding type power supply that has a converter output circuit is described in 20090230941 (Vogel). All three patents are owned by the owner of this patent, and all three of which are hereby incorporated by reference. Other welding type power supplies include additional stages, or use other topologies for each stage (such as a buck preregulator, a combined boost rectifier for a preregulator, a chopper instead of or following the inverter, a second inverter, etc.
Boost preregulators offer many advantages, including the ability to receive a range of inputs, the ability to power factor correct, and the ability to provide a high voltage bus which allows for a smaller output transformer, but they do have some drawbacks. For example, the losses in switches can be significant in a welding power supply, particularly when they are hard switched.
The power loss in a switch at any time is the voltage across the switch multiplied by the current through the switch. Hard switching turn-on losses occur when a switch turns on, with a resulting increase in current through the switch, and it takes a finite time for the voltage across the switch to drop to zero. Soft switching attempts to avoid turn-on losses by providing an auxiliary or snubber circuit with an inductor in series with the switch that limits the current until the transition to on has been completed, and the voltage across the switch is zero. This is referred to as zero-current transition (ZCT) switching.
Similarly, hard switching turn-off losses also occur when a switch turns off, with a resultant rise in voltage across the switch, and it takes a finite time for the current through the switch to drop to zero. Soft switching attempts to avoid turn-off losses by providing an auxiliary or snubber circuit with a capacitor across the switch that limits the voltage across the switch until the transition to off has been completed, and the current through the switch is zero. This is referred to as zero-voltage transition (ZVT) switching.
Often it is not practical or cost effective to use a true ZCT and ZVT circuit. U.S. Pat. No. 6,115,273 (Geissler) describes an approximation using slow voltage/current transitions (SVT and SCT). SVT and SCT as used herein, describe transitions where the voltage or current rise is slowed (rather than held to zero), while the switch turns off or on. While U.S. Pat. No. 6,115,273 teaches an improvement over the prior art, the circuit that is used to provide the SVT and SCT transitions can be stressed during the transitions. Particularly, diode D4 of FIGS. 5-13 is stressed.
Accordingly, a welding type power supply with a preregulator having a switch that with SVT and SCT switching and with protection for the components used perform the SVT and SCT is desirable.